you don't mention which part of device is not working, and the fan merely pushes basic air into cab, there are two types of fan systems , one is for high pressure A/C and Heater/defrost, the other is basic heater / air flow,

You turn on A/C and open hood to see if compressor is turning with belt drive, if not, then may be relay fuse switch,
if it is turning then that is the Freon gas leaking or low or not available, but the pipe as well should get cold or frost condensation,

then double check that you have fan switch on and the A/C button on, and set to defrost or High Air or High heat, if set to midway, it may not work as you are not switched to Air that computer knows,

and reminder that heater defrost is same as air in that it turns on Air to create more engine heat form heat faster than normal cab heat.

If it suddenly stopped working, you may (if you're lucky) just have a leak that needs sealing. On the other hand, the compressor may be shot which will be expensive. If it gradually got worse, it may just need regassing especially if it's never been done since new.